What advice can you give regarding my wife who has dementia and temper tantrums?

Two reactions that are common for people with dementia are fear and anger. The emotions are pretty much flip sides of the same fight-or-flight coin. If someone has fear it is best to try to reassure them as well as possible. If someone has anger it is often best just to walk away. In my mother's case, her anger is usually because she feels out of control or wants me to do something I don't want to do. In this situation I try to let it slide and walk away until things smooth over. Arguing with her or talking back doesn't do anything but frustrate me and make me feel bad.

Has she been seen by a geriatric psychiatrist? Sometimes antidepressants and/or antianxiety meds can make a huge difference.

Is this a sudden change in mental status? If so, she should be checked for a UTI.